Electrophotographic marking is a well-known method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired final image onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges so as to produce an electrostatic latent image of the desired image on the photoreceptor's surface. Toner particles are then deposited onto that latent image so as to form a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a black and white electrophotographic marking machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, called the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner to produce a black toner image. The recharge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various latent images and consequently the color toners are placed in a superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
The REaD IOI process can be performed in a various ways. For example, in a single pass printer wherein the composite image is produced in a single pass of the photoreceptor through the machine. This requires a charging, an exposing, and a developing station for each color of toner. Single pass printers are advantageous in that they are relatively fast since a color image is produced during each cycle of the photoreceptor.
One way of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is comprised of a laser light source (or sources) and a rotating polygon having a plurality of mirrored facets. The light source radiates a laser beam onto the polygon facets. That beam reflects from the facets and strikes the photoreceptor, producing a light spot. As the polygon rotates the spot traces lines, referred to as scan lines, on the photoreceptor. The direction of the sweeping spot is called the fast scan direction. By moving the photoreceptor perpendicular to the fast scan direction, as the polygon rotates the spot raster scans the photoreceptor. The direction of motion of the photoreceptor is referred to either as the slow scan direction or the process direction. During scanning, the laser beam is modulated to produce the desired latent image.
In color electrophotographic printing it is very important that the various color latent images be accurately registered with each other. By registration it is meant that the latent images are produced such that when the various latent images are developed and transferred that the desired composite image results. Latent image misregistration causes color errors that are highly noticeable by the human eye.
Various factors cause misregistration. For example, photoreceptor motion may not be perfect because vibration, motor backlash, gear train interactions, mechanical imbalances, and/or friction, among other factors, can cause the instantaneous position of the photoreceptor to be less than ideal. Another cause of misregistration is polygon facet misplacement, also referred to as phasing errors. Facet misplacement comes about because it is very difficult to accurately synchronize the rotation of the polygon with the motion of the photoreceptor. When the photoreceptor is in the proper position to receive the latent image the polygon facet that should reflect the laser beam might be +Y2 a scan line off. The result is either a delay or an advance of the first scan line of a latent image, which causes the remaining scan lines to be offset.
Misregistration in the fast scan direction is usually reduced by using a start-of-scan sensor in the optical path. That sensor detects when the sweeping spot is at a predetermined location. Using that information the modulation of the laser beam is controlled such that the latent image starts at the correct place on the photoreceptor. However, misregistration in the slow scan direction is harder to control. One approach is to accurately control the photoreceptor's motion. However, because of inertia, backlash, and other mechanical motion problems this is difficult and expensive to do. As previously noted, misregistration caused by phasing errors is also very difficult to control.
Another approach to reducing misregistration is to use "aerial" image control. In aerial image control the scan line itself is adjusted to prevent or reduce misregistration. For example, U.S. Pat. No. 5,287,125 to Appel et al. discloses a raster output scanner that has process direction (slow scan direction) can line position control. In that patent an error feedback circuit senses the position of a moving photoreceptor. Photoreceptor position errors are used to produce signals that are applied to a piezoelectric actuator. The piezoelectric actuator expands or contracts, moving a pre-polygon lens, which moves the scan line produced on the photoreceptor so as to correct for photoreceptor motion errors. Additionally, U.S. Pat. No. 6,023,286 entitled "MOVING MIRROR MOTION QUALITY COMPENSATION," filed on Dec. 31, 1997 by Nowak et al. teaches a mirror moved by a piezoelectric element to correct for photoreceptor motion errors.
While the references cited above are useful, they have their limitations. In U.S. Pat. No. 5,287,125 connecting a pre-polygon lens to the piezoelectric actuator such that the pre-polygon lens accurately tracks rapid expansions and contractions of the actuator, while simultaneously providing accurate positioning of the pre-polygon lens for proper focus, is difficult and expensive. Furthermore, the scan line movers taught in U.S. Pat. No. 5,287,125 and in U.S. Pat. No. 6,023,286 have mechanical factors that limit the rate at which scan line adjustments can be made. This prevents them from being used to correct for phasing errors.
Therefore, a scan line adjustment technique capable of accurately adjusting the scan line position to correct for both photoreceptor position errors and for phasing errors, would be beneficial. Even more beneficial would be such a scan line adjustment technique that can be implemented at relatively low cost.